Patent GB-661955-A (GB)
Title: Telecommunication switching systems and exchanges
IPC: H04Q11/04
Filed: 1948-04-23
Abstract: 661,955. Automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd. April 23, 1948 [April 26, 1947], No. 11181/48. Class40(iv). An electronic distribution system of the type described in Specification 647,773 is provided with an electronic register-controller which is associated with each link only during the setting up of the call. The calling subscriber's pulses produced at instants tc in the distributer cycle are shorn of their modulation and then applied over lead 24 to the gate valves 23 of all the line-finders. Fig. 4, which are of the lock-in oscillator type described in Specification 647,773. When a line-finder is seized the incoming pulses are clipped by grid current in valve 23 to prevent them seizing other linefinders. Pulses obtained from valve 23 are applied via valve 50 in the selector circuit, Fig. 5, and the common cathode follower 52 to the input gates 59 of all the register-controllers, Figs. 2, 3, 6, one of which is seized in a manner similar to that of a line-finder. Pulses from valve 59 are then applied to valve 48 to admit the calling subscriber's modulated pulses and pass them to the digit receiving equipment Figs. 3, 6. When these pulses are modulated with dial impulses, the latter are registered and pulses are produced by valve 110 at the called subscriber's instant ta in distributer cycle, as described in Specification 647,773. Each selector circuit is associated with a different dynode of a comparatively slowly sweeping link identification distributer tube 8 which produces, over lead 133, pulses of duration equal to 20 sweeps of the distributers to which the subscribers are connected, which pulses are fed to the gate valves 128, 134 of the selectors. During periods ti of the sweep of distributer 8 appropriate to the selector in question pulses at instants tc, fed from valve 50 to the valve 134, are transmitted in bursts of 20, via the common cathode follower 136 to the gates 139 of all the register-controllers. The valve 139 of the register-controller in use gates these pulses at instants tc by pulses obtained from valve 48, and its output is integrated to produce continuous pulses during the period ti. These open the gate 124 (provided the called subscriber is free) which thereupon passes bursts of 20 pulses at instants ta during the periods ti via the common valve 126 to the gates 128 of all the selectors. Only the gate 128 of the selector in use is unblocked by pulses from the distributer 8 and when pulses derived from the lock-in oscillator 144 drift into coincidence with pulses at instants ta, this valve conducts to change over the states of valves 150, 151, thereby causing valve 162 to admit current from a standard source to lock the oscillator 144. The changeover of valves 150, 151 also blocks valve 50 to release the register-controller. The called subscriber's input and output gates 158, 159 are opened by pulses at instants ta derived from valve 149, and the calling subscribers input and output gates 44, 45 by pulses at instants tc derived from the line-finder gate 23, communication connections being established between appropriate gates over low-pass filters. Valves 150, 151 further provide a pulse to change over valves 165, 166 to open the ringing gate 167 to pass ringing current to the called subscriber's output gate. When the called subscriber answers pulses from the called subscriber's input gate 158 restores 165, 166 to cut off the ringing current. Pulses at instants tc are applied to cause valve 176 to conduct to neutralize the answering subscriber's pulses on lead 24 which would otherwise seize another line finder. Pulses derived from tube 116, which is fed both by calling subscriber's pulses directly and also by pulses at called subscriber's instants obtained from the selector circuits, are compared by gate 118 with pulses at the called subscriber's instant ta, derived from valve 110 and if coincidence is registered, i.e. the called subscriber is busy, the register circuit is prevented from moving to the speaking condition and gate 124 remains closed on its suppressor grid and the selector circuit is not seized.